The present invention relates to a system to provide sufficient and comfortable lighting within a space. In particular, the invention relates to a system for the automatic control of the light levels in a space by the control of the intensity of electric lighting and/or daylight in a space. In particular, in one embodiment, the present invention is directed to the control of the lighting level in a space, such as an interior room, by controlling both the artificial light in the space by control of the intensity of electric lighting in the space and the control of motorized window treatments in the space in order to achieve a reasonably constant illumination on task surfaces throughout the space. In addition, the invention is directed to a system to reduce or prevent sun glare, which can potentially occur at low sun angles due to sunshine through windows or other openings, e.g., skylights, surrounding the space. Such a condition is likely to occur at or near sunset or sunrise.
Further, the invention is directed to the control of electric lighting in a space in multiple zones of the space to achieve a preset lighting profile in the space. A “lighting profile” represents a desired distribution of target illumination values in various portions of the space. Additionally, the invention is directed to the control of window treatments such as shades based on light levels in the interior of the space so as to maintain a predefined illumination profile in the space and/or to minimize or eliminate sun glare through openings into the space. Further, the invention is directed to a system which performs the three functions of controlling electric lighting in the space, controlling natural lighting in the space in order to achieve a predefined illumination profile and minimizing or eliminating sun glare into the pace. The invention is thus directed to an illumination maintenance system for achieving a predefined illumination profile in a space where the light is provided by natural light or artificial light or both and further where sun glare is optionally minimized or eliminated.
One of the major problems of illumination maintenance systems, and in particular, closed loop (feedback) illumination maintenance systems, is the variation of incident light at the sensor or sensors employed for detecting the incident light due to occupants moving in the space or some other type of variation of surface reflections in the space. One of the prior art approaches to solve this problem is to average the illumination readings from multiple light level sensors. Another approach is to position or orient the field of view of the sensors such that the sensors are not influenced by the occupant traffic or other short or long term variations of the optical properties of the environment.
Further, open loop systems have been developed for illumination maintenance and daylight harvesting but such open loop systems are not suitable for window treatment control implemented based on the interior light sensors because when a shading or window treatment device is closed, access to exterior lighting conditions is prevented or restricted.
Currently available commercial solutions for daylight control of window treatments are mostly based on exterior light sensors and predictive control algorithms. Exterior light sensors cause maintenance problems and require exterior wiring. Predictive control schemes are difficult to configure. Usually a long process of measurements and computer or mechanical model simulations must be performed before the control system can be correctly configured.
Further, a conventional approach that attempts to solve the glare problem due to sunshine entering through windows at a low sun angle utilizes some form of open loop control of window treatments. In these systems, the algorithms are usually based on the use of exterior photosensors. These conventional systems employ a combination of strategies based on the exterior light level readings and a time clock in order to derive the required shade positions. A study of the expected lighting conditions is regularly performed in order to predict the times when the glare incidents are likely to occur. Some of the problems with this type of control are that it demands maintenance of exterior photo sensors exposed to the elements and there are problems with wiring and/or mounting sensors continuously exposed to the outside lighting conditions. Furthermore, preparation and creation of complex databases is required to define the lighting conditions for each space of a building throughout a year for large buildings, which is time consuming and expensive. Further, these systems require control database modifications in case exterior shading objects are added such as new buildings or plants and further, the controls cannot be fully optimized for each space of a large building and therefore do not result in optimal occupant comfort and energy savings.